Many communication systems have a configuration characterized by a plurality of stations interconnected by a common transmission channel, and information interchange among the stations occurs on a time-shared basis over the channel. In these systems, conflicts may arise whenever two or more stations simultaneously request use of the channel for transmission.
For these so-called contention situations, methods and concomitant arrangements exist for dealing with conflicts and these vary in complexity from simple, fixed priority techniques to sophisticated code division multiple access procedures.
In general terms, a contention period comprises: a duration wherein a plurality of stations submit requests for allocation of the channel; an interval to detect the multiple requests; and a duration to resolve the conflict and allocate the channel to the selected station. During the contention period, no actual or message-bearing communication may occur, thereby resulting in reduced utilization of channel capacity. From another viewpoint, if a normalized efficiency per transmission period is defined as the ratio of the time utilized for information-bearing message transfer to the total time period, then the efficiency is generally less than one for conventional contention resolving systems.
As a specific example of a technique for arbitrating contentions, the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) media access method is briefly described; it is considered illustrative of the more sophisticated contention resolution methods. (The CSMA/CD method is discussed in detail in the text entitled "Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications", published by The Institute of Electrical and Electronic Engineers, Inc. in 1985). To transmit, each station monitors for a quiet period on the channel by detecting that no other stations are transmitting, and then emits the intended message in bit-serial form. If, after commencing transmission, the message collides with that of other stations, then each transmitting station intentionally sends additional bits to ensure propagation of the collision throughout the system. The stations then deactivate for a random time period, called the backoff time, before attempting to transmit again. This process continues until only a single message propagates collision-free over the channel. However, as is discernible even from this brief overview, this technique is somewhat inefficient due to station deactivation during the backoff periods and the random nature of the retry process.
Other, more specialized techniques have been disclosed to deal with contention on a more deterministic basis. Representative of these techniques are the disclosures in U.S. Pat. No. 3,796,992 and Great Britain Pat. No. 1,365,838.
In Great Britain Pat. No. 1,365,838, the particular system considered is one in which a number of data handling devices (slave stations) are in communication with a common controller (master station) over a bus and any one of the devices may require service from the controller at any given time. At system initialization, each data handling device is assigned a fixed interrupt number indicative of the device address. The controller solicits interrupt requests by transmitting a coded message. Each device, if requesting use of the bus, responds by serially transmitting its interrupt number simultaneously with other devices. Then each device compares the bus content bit-by-bit and dynamically either terminates or continues its transmission according to the results of the comparison. With such a technique, the time required to resolve a contention is fixed by the number of bits assigned to the interrupt numbers. However, even though this time is deterministic, there is still a period in which no actual information-bearing messages may be transmitted and the normalized efficiency is less than one.
None of the conventional techniques disclose or suggest a methodology for always achieving the optimum normalized efficiency as defined above. Moreover, none of the known methods suggest the utilization of more than one channel or bus to resolve or even eliminate contention situations.